The present invention relates to electrostatic ink jet recording apparatuses that achieve stable print density.
The ink jet method is an inexpensive, high quality, and high speed printing technology, and is extensively applied to copying machines, facsimile machines, printers, and word processors as an office-use and personal-use recording apparatus.
Ink jet recording systems include types which use an electric-heat converting element such as an exothermic resistor; an electric-mechanical converting element such as a piezo element; and the electrostatic type using electrical energy directly as an energy generator for ejecting toner.
In particular, the manufacture of a recording head for the electrostatic ink jet system is simple compared to those of other systems, and area-wide grayscale recording is achievable by controlling the electrical signals applied to the recording electrode. In addition, the current consumed during recording is extremely small, making it a promising recording technology with respect to energy saving. Furthermore, the use of oil pigment inks offers highly water-proof printing results, which are particularly convenient in an office environment.
The principle of the electrostatic ink jet system is briefly described next.
The Japanese Laid-open Patent No. S56-4467 discloses the principle of ejecting and propelling ink drops from the recording electrode to a counter electrode by applying a few kilovolts of voltage between the recording electrode, which contains the ink, and the counter electrode holding a recording medium. When the electrostatic force applied to the ink becomes higher than the surface tension of the ink, ink drops are ejected and travel from the recording electrode towards the counter electrode.
The typical configuration of a head based on this principle is disclosed in the U.S. Pat. No. 4,271,416 and the Japanese Laid-open Patent No. S56-4467. The disclosed configuration enables the use of a slit for ink ejection shared by multiple recording electrodes which removes the need for a nozzle hole for each recording electrode. The chief advantage of this nozzle-less configuration is the reduction of clogging by dried ink. Accordingly, this configuration is extremely effective for line heads having a recording electrode equivalent in length to the width of the recording medium, as well as a serial head in which some hundreds of recording electrodes scan the recording medium in the width direction.
In the above electrostatic ink jet system having two or more recording electrodes, a closed loop with an adjacent electrode may be created when operating each recording electrode independently. However, the closed loop may result in potential leakage which needs to be prevented. For this purpose, oil-based solvents with high electrical resistance are generally used as ink solvents. The Japanese Laid-open Patent No. S58-215353 uses oil-based ink with a specific resistance of about 108 cm, surface tension 18 of dyne/cm, viscosity of 2-30 cP, and specific gravity of 1.0 g/cm3. Oil-based pigments or oil-based dyes may be used as toners, but oil-based pigments are used more commonly because of their wide variety of types.
However, in the electrostatic ink jet using the above pigment and oil-based solvent, a certain amount of solvent is ejected together with the pigment when ejecting the ink. This leads to limited print density caused by an undesirable ratio of electrostatic charge applied between toner particles and solvent.
The Japanese Laid-open Patent Nos. H8-295023 and H9-19389 attempt to solve this problem by controlling the ratio of force applied to toner particles and solvent with the aim of ejecting only toner particles in the ink. In these prior arts, the ejecting principle is not completely described, but the above problem appears to be solved by applying electrostatic force only to the toner particles dispersed in the solvent with the aim of ensuring that only the toner particles are ejected and that no solvent accompanies them in ejected ink. In order to achieve the independent movement of the toner particles and solvent, the toner particles need to be dispersed rather than dissolved in the solvent. This is because that if the toner is dissolved, the toner particles and solvent cannot be distinguished to then selectively eject only the toner particles.
The Japanese Laid-open Patent No. H8-295023 applies a voltage with the same polarity as the polarity of the charge of the toner particles to the recording electrode so that only the toner particles migrate and are attracted to the tip of the recording electrode by electrostatic repulsion. Ink drops containing a high density of toner particles are then ejected.
However, as described in the Japanese Laid-open Patent No. H10-52920, the electrostatic repulsion from the recording electrode applies a three-dimensional repulsion to toner particles. This results in the toner particles being attracted to the meniscus in areas where a meniscus is present, but, on the other hand, results in dispersion of toner particles in other areas distant from the meniscus. Consequently, toner particles to be supplied in the next ejection becomes insufficient, causing an extremely low toner particle density at the ink ejection opening, and thus markedly reducing the print density. In extreme cases, toner particles may not be ejected at all. This problem may occur when there is a single recording electrode, but becomes more serious when a so-called slit head having two or more recording electrodes and a common ink ejection opening is used. The problem becomes even more critical with increasing width of the ink ejection opening. A so-called full-line head whose ink ejection opening is long in the width direction of the recording medium may suffer this problem particularly obviously. This is because the dispersion is more likely to occur when the space allowing dispersal of toner particles is larger.
In order to solve the above problem, the Japanese Laid-open Patent No. H9-156106 proposes the following method: A potential difference is applied between the recording electrode and its adjacent electrode for attracting toner particles near the recording electrode, and the recording signals are supplied to the recording electrode while the toner particles are being attracted.
FIG. 5 is a perspective view of an electrostatic ink jet head in a preferred embodiment of the present invention. A conventional control system is described with reference to FIG. 5 for convenience. FIGS. 10 and 11 are timing charts illustrating the control system of a conventional electrostatic ink jet head.
In FIG. 5, the ink jet head comprises recording electrodes 11, collecting electrodes 12, upper plate 4, and lower plate 5; and contains ink 6. Electrodes selected for printing in recording electrodes 11 are hatched. A higher potential is applied to collecting electrode 12 compared to that applied to recording electrode 11 for attracting positively charged pigment to recording electrode 11. The positive pulse signal applied to recording electrode 11 enables ink to be ejected from ink ejection opening 8 as a result of electrostatic repulsion.
In FIG. 10, signal 301 is applied to the recording electrodes for printing. Signal 302 is applied to the collecting electrodes. Signal 303 is applied to a counter electrode. In FIG. 11, signal 304 is applied to the recording electrodes not being used for printing.
In FIG. 10, the electrostatic potential (signal 302) of the collecting electrodes is High while the voltage of the recording electrodes (signal 301) is Low so that positively charged pigment is attracted to and migrates towards the recording electrodes. Then, when the voltage of the recording electrode is raised, the pigment attracted to the recording electrodes for printing is ejected and travels from the recording electrode by electrostatic repulsion. On the other hand, since the High signal is not applied to the recording electrodes not being used for printing (signal 304 stays low), the ink is not ejected from these recording electrodes.
This method, however, still causes dispersion of toner particles during the ON period when the voltage of the recording electrodes for printing is High, due to there being small or no potential difference between the recording electrodes and collecting electrodes during this period. This may inhibit the improvement of the recording speed, disable the ejection of large dots, and prevent stable ejection.
The present invention aims to offer an electrostatic ink jet recording apparatus for solving the above problems in the prior art.
The electrostatic ink jet recording apparatus of the present invention comprises:
(a) a first electrode;
(b) a second electrode; and
(c) a third electrode.
Potential difference is selectively applied between the first and second electrodes for selectively attracting toner particles by electrostatic force, and the ink is ejected and travels to print on a recording medium by giving potential to the third electrode.
This configuration solves the conventional problems caused by dispersion of the toner particles while applying the recording signals, including decreased recording speed, no ejection of large dots, and unstable ejection.